Right, basically it looks like the actual regulation (rather than what's stated on the page you link to) is sufficiently wooly that it can't really be brought to hold a provider to account unfortunately. I'm not an expert, but the relevant documentation is at http://www.ofcom.org.uk/static/archive/oftel/publica...
That is a long obsolete version of the General Conditions - especially General Condition 18 on number portability. There's been some extensive sets of changes since then, not least because of changes in the underlying EU law. The latest consolidated version of the General Conditions can be found on the
Ofcom General Conditions web page.
There's little ambiguity in the latest version of General Condition 18.1:
18.1 The Communications Provider shall provide Number Portability within the shortest possible time, including subsequent activation, on reasonable terms and conditions, including charges, to any of its Subscribers who so request.
As was repeatedly drummed into me in university law lectures, any lawyer is cautious over the word "reasonable", as it introduces subjectivity - but there is no doubt that there is a right to portability.
As orbrey says, the right is the subscriber's to enforce - but I'm sure Ofcom would be interested if a major provider like Sky is refusing requests to port out.